Recently, with the spread of personal computers, a digital still camera and a digital video camera (hereinafter referred to simply as “digital cameras”) which can take image information easily into a digital device are coming into wide use on an individual user level. The digital camera of this kind is expected to grow popular more and more as an input device for image information, in the years to come.
Further, downsizing of a solid-state image pickup element such as CCD (Charge Coupled Device) carried by a digital camera has been advanced, whereby, the digital camera is also required to be further downsized. Therefore, an image taking lens that occupies the largest space in a digital input device is also required strongly to be compact.
The easiest way to downsize an image taking lens is to make a size of the solid-state image pickup element small, and for this reason, a size of a light receiving element is required to be small. As the result, it increases a difficulty of manufacturing the solid-state image pickup element and increases performance to be required for the image taking lens.
On the other hand, when a size of the image taking lens is made small with a size of the solid-state image pickup element unchanged, a position of an exit pupil gets nearer to an image plane inevitably, and an off-axis light flux emerging from the image taking lens enters an image plane obliquely. As a result, it becomes difficult for the light-converging capability of a micro-lens provided on the front surface of the solid-state image pickup element to be exhibited sufficiently, resulting in a problem that an extreme difference in terms of image brightness is observed between a central portion of the image and a peripheral portion of the image. For this reason, a position of an exit pupil of the image taking lens needs to be arranged to be away from an image plane, which makes it difficult to avoid a large-sized image taking lens.
Further, in recent years, it has been required to manufacture at low cost an image taking lens capable of coping with high density of a solid-state image pickup element.
For the aforesaid requirement, there has been disclosed an image taking lens for a solid-state image pickup element provided with three lenses whose power (which expresses an amount defined by with an inverse number of a focal length) are made to be positive, negative and positive respectively and an aperture stop arranged on closest to the object side of the image taking lens (for example, see U.S. Patent Application Publication Nos. 2003/0193605, 2004/0212901 and 2005/0002116, U.S. Pat. No. 6,977,779, Japanese Patent Application Publication Nos. 2004-219807, 2004-317743 and 2005-10622). There is further disclosed an image taking lenses for a solid-state image pickup element provided with three lenses whose powers are made to be positive, negative and negative respectively and an aperture stop arranged between the lens with the positive power and the lens with the negative power (for example, see U.S. Patent Application Publication No. 2005/275950 and U.S. Pat. No. 6,980,372).
However, the former conventional image taking lens provides only one negative lens as the second lens (a lens that is second from the object side), and power of the negative lens becomes too strong. It is therefore estimated that sensitivity against manufacturing errors for the lens optical axis becomes high, and it is difficult to manufacture while keeping peripheral optical performance.
Further, since the latter conventional image taking lens provides an aperture stop located inside the optical system, a distance from the image plane to the position of an exit pupil for the total image taking lens is too short, therefore, the aforesaid image taking lens is not preferable as an image taking lens for a solid-state image pickup element.